1. Field of the Invention
The present invention relates to a porous membrane made of a vinylidene fluoride resin, which is excellent in fractionating capability, permeating capability and is suitable for use in the field of water treatment such as, for example, water purification, preparation of drinking water, preparation of water for industrial use, wastewater treatment, food engineering, charged membranes and fuel cells and also to a method of making such porous membrane which is excellent in aspects of production controllability, cost and pore forming characteristics.
2. Description of the Prior Art
In recent years, the technology of separating means utilizing a porous membrane having a selective permeability has been remarkably developed. Such a separating technology is utilized in practice as a series of purifying systems, including separating process, cleansing process and sterilizing process, in the field of, for example, manufacture of drinking water, ultra-pure water and medicines and microorganism elimination and finishing of brewed products. In those fields of application, fining (a high level processing) of water and increase of safety and increase of precision are required at a high level and the use of porous membranes is being widely spread. In view of those situations, required characteristics of the porous membrane have come to be sophisticated. Of those membrane characteristics, the permeating capability and the fractionating capability are considered particularly important. As far as the permeating capability and the fractionating capability are concerned, they must be balanced relative to each other so that particles as small as possible can be removed at a high rate of pure water permeate flow and, in order to achieve this, the continuity of pores and the membrane surface structure are important.
In the water purifying treatment, for the purpose of sterilization of permeate and prevention of bio-fouling in the membrane, a disinfectant such as, for example, sodium hypochlorite is added to a membrane module portion and/or the membrane itself is rinsed with acid, alkali, chlorine or surface active agent, and, accordingly, the porous membrane is also required to have a chemical resistance. In the preparation of tap water, since pathogenic organisms tolerant to chlorine such as, for example, cryptosporidium originating from livestock excrement and urine cannot be sufficiently removed in the water filtration plant and, hence, troubles associated with admixture of pathogenic organisms in the treated water have come to be elicited since 1990, the demands have arisen that the porous membrane must have a sufficient separating or screening characteristic effective to prevent an admixture of raw water into the treated water and, also, a high physical strength enough to avoid thread breakage.
As discussed above, some important characteristics required for the porous membrane to have includes the separating or screening precision, permeating performance, physical strength and chemical strength (chemical resistance). Accordingly, development of a porous membrane with the utilization of polyvinylidene fluoride has recently come to be advanced. The porous membrane utilizing the vinylidene fluoride resin is excellent not only in strength, elongation and chemical resistance, but also in resistance to oxidant and can, therefore, be utilized in a high level water treatment using ozone that has recently gained public interest.
As hereinabove discussed, the physical strength of the porous membrane and the chemical strength thereof represented by the chemical resistance are mostly attributable to the characteristics of material used to form the porous membrane, but the permeating and fractionating performances of the porous membrane depends on the method of preparing such porous membrane.
As a method of making a hollow fiber membrane capable of realizing a high rate of pure water permeate flow, a stretch opening method may be enumerated. This method is characterized in that material for the membrane is annealed and stretched under a specific condition and, as a result, a hollow fiber membrane having a multiplicity of slit shaped micropores formed therein having been bounded by microfibrils and nodes defined by stacked lamellae can be manufactured. (See, for example, the Patent Document 1 quoted below.) However, the hollow fiber membrane so manufactured according to the method referred to above involves such a problem that since as a result of stretching fibers are oriented in a direction of fibers, the strength in a circumferential direction tends to be reduced considerably. In particular, since the strength tends to be reduced as the pore size increases, and, therefore, it is difficult to manufacture with the method referred to above, the hollow fiber membrane having a practically acceptable strength. Also, since although the pure water transmission rate is high, the pore size is largely distributed and the pores so formed represents a slit-like shape, there is a problem that substances of an elongated shape tends to be easily transmitted, resulting in reduction in separating or screening precision.
As a method of manufacturing a separating membrane excellent in permeating and fractionating performances, the case in which phase separation is utilized is largely well known. The membrane manufacturing method utilizing such a phase separation can be generally classified into a non-solvent induced phase separation process and a thermal induced phase separation process.
In the non-solvent induced phase separation process, a uniform polymer solution containing a polymer and a solvent mixed together brings about a phase separation as a result of change in concentration induced by ingress of a non-solvent and evaporation of a solvent to external atmosphere. As a method of manufacturing a separating membrane with the use of such a non-solvent induced phase separation process, the method is known, in which after a polysulfone resin is dissolved in a solvent such as, for example, N,N-dimethyl acetamide or the like, non-solvent induced phase separation is initiated within a coagulation bath to thereby form the separating membrane. (See, for example, the Patent Document 2 quoted below.) However, the non-solvent induced phase separation process generally has such problems associated with the physical properties of the membrane, the process controllability and the cost because it is difficult to control the phase separation within the non-solvent, the cost of manufacture is high because of the necessity of use of the non-solvent, and macrovoids tend to occur.
On the other hand, the thermal induced phase separation process generally includes the following steps.
(1) A mixture of a polymer and a solvent having a high boiling point is melted at high temperature;
(2) Cooling is carried out at a proper rate in order to induce the phase separation to thereby solidify the polymer; and
(3) The solvent used is extracted.
Advantages of the thermal induced phase separation process as compared with the non-solvent induced phase separation process are as follows:
(a) Formation of macrovoid, which constitute a cause of reduction in strength of the membrane, can be prevented. (b) With the non-solvent induced phase separation process, the use of a non-solvent other than a solvent is needed and, therefore, a control is difficult to achieve during the manufacture accompanied by reduction in reproducibility. On the other hand, with the thermal induced phase separation process, the use of the non-solvent is not needed and, therefore, the process controllability and the cost characteristic are excellent, accompanied by a high reproducibility. (c) Control of the pore size can be relatively easy to achieve and distribution of the pore size is so sharp as to bring about an excellent pore forming characteristic.
The thermal induced phase separation includes a solid-liquid type thermal induced phase separation and a liquid-liquid type thermal induced phase separation and which one of them develops depends on the compatibility between the polymer and the solvent. Where the compatibility between the polymer and the solvent is very high, the solid-liquid type thermal induced phase separation will develop, but when such compatibility is lowered, the liquid-liquid type thermal induced phase separation develops and the both will eventually become incompatible with each other. In general, since in the liquid-liquid type thermal induced phase separation proceed, the phase separation proceeds as a result of the spinodal decomposition, the liquid-liquid type head induced phase separation has such a feature that the co-continuous structure tends to develop easily as compared with the solid-liquid thermal induced phase separation and, as a result, it is possible to manufacture the separation membrane excellent in excellent pore forming characteristics such as, for example, pore continuity and uniformity. In other words, in order to manufacture the separation membrane excellent in permeating performance and fractionating performance, it is desirable to select a proper combination of a polymer and a solvent that leads to development of the liquid-liquid type thermal induced phase separation. However, since the region in which the polymer and the solvent lead to development of the liquid-liquid type thermal induced phase separation is limited, selection of a proper combination of the polymer with the solvent is known to be important where the separation membrane is desired to be manufactured with such method referred to above. (See, for example, the Non-patent Document 1 quoted below.)
The method of making the porous membrane made of a vinylidene fluoride resin, which utilizes the liquid-liquid type thermal induced phase separation process is well known in the art. (See, for example, the Patent Document 3 quoted below.) However, the porous membrane so manufactured with such process is equal to or higher than 30000 L/m2/hr/98 kPa and a fractionated particle size not smaller than 1 μm, a series of examinations made by the inventor of the present invention has revealed that such porous membrane contain a number of pores that are not communicated with each other, that is, closed pores and, yet, the pores of the substantially same size appear on opposite surfaces of the membrane and, therefore, such porous membrane is incapable of exhibiting a high rate of pure water transmission relative to the fractionated particle size.
Also, since the vinylidene fluoride resin is hydrophobic, water will not permeate through the membrane of polyvinylidene fluoride unless it is treated otherwise, and, therefore, a hydrophilizing treatment is required in order for a hydrophilic liquid including water to penetrate through the membrane.
The following literature is available as patent documents believed to be relevant to the present invention.                Patent Document 1: JP Laid-open Patent Publication No. H05-49878, published Mar. 2, 1993.        Patent Document 2: JP Laid-open Patent Publication No. H11-104235, published Apr. 20, 1999.        Patent Document 3: JP Laid-open Patent Publication No. 2005-194461, published Jul. 21, 2005.        Patent Document 4: JP Laid-open Patent Publication No. 2003-138422, published May 14, 2003.        Patent Document 5: JP Laid-open Patent Publication No. 2005-200623, published Jul. 28, 2005.        Non-patent Document 1: “Chemical Engineering” published by Kagaku Kogyosha, the issue of June, 1998, pp. 453 to 464.        